An international workshop on the use of rodent tumors in experimental cancer therapy is planned for October 21-24, 1984. It is to be attended by a strictly limited group of 60 investigators who are active in experimental radiotherapy, chemotherapy, and immunotherapy. This workshop will attempt to reach agreement on practices that could be followed by everyone who works with tumors in laboratory animals. There will be exhaustive discussion of principles and practices to be followed in experimentation with tumors in animals, in order that such experiments be maximally definitive and unambiguous. The agenda will ensure coverage of the following topics: Assays of therapeutic effectiveness by tumor control (TCD50), tumor regrowth, and excision assay (both in vivo and in vitro); choice of suitable tumor lines and their maintenance; suitable host animal strains; suitability of xenografts; and statistical principles and details.